java 归并排序求逆序数，先解决相邻数字之间的逆序，再回溯

class Solution {
    public int ans = 0;

    public int inversePairs(int[] nums) {
        int n = nums.length;
        int[] helper = new int[n];
        merge(nums, helper, 0, n - 1);
        return ans;
    }

    private void merge(int[] nums, int[] helper, int left, int right)
    {

        if (left >= right)
            return;
        int mid = left + (right - left) / 2;
        merge(nums, helper, left, mid);
        merge(nums, helper, mid + 1, right);
        for(int i = left ; i <= right; i++)
        {
            helper[i] = nums[i];
        }

        int beginLeft = left;
        int beginRight = mid + 1;
        int current = left;
        while(beginLeft <= mid && beginRight <= right)
        {
            if(helper[beginLeft] <= helper[beginRight])
            {
                nums[current] = helper[beginLeft];
                beginLeft ++;
                current++;
            }else{
                nums[current] = helper[beginRight];
                beginRight++;
                ans += (mid - beginLeft +1);
                current++;
            }
        }
        while(beginLeft <= mid)
        {
            nums[current] = helper[beginLeft];
            beginLeft ++;
            current++;
        }
        while(beginRight <= right)
        {
            nums[current] = helper[beginRight];
            beginRight ++;
            current++;
        }
    }
}